Pumps of the type shown in U.S. Pat. NO. 4,029,442 are well known for use in pumping abrasive and highly viscous materials. The contents of the said U.S. Pat. No. 4,029,442 are herein incorporated by reference. While the annular ring seal shown therein is highly effective in sealing against the piston and allowing that pump to perform, the piston and seal are subject to wear due to the highly abrasive materials which may be pumped. In particular, the seal typically used in commercial devices such as that referenced above, is generally made of a relatively hard nylon material. The wear mechanism in such seals results from abrasive particles becoming imbedded in the hard seal material, thus turning the seal into a composite of hard abrasive particles with a tough plastic which retains and supports those particles. It is primarily an object of this invention to provide an improved sealing mechanism which in particular extends the wear life of the piston and seal.